A weekly or even regularly scheduled full crawl, normally, isn't necessary.
Based on Microsoft's recommendations ([Article][1]), you would only need to manually do a full crawl after your initial full crawl in the following cases (FTA):
One or more hotfix or service pack was installed on servers in the farm. See the instructions for the hotfix or service pack for more information.
An SSP administrator added a new managed property.
To re-index ASPX pages on Windows SharePoint Services 3.0 or Office SharePoint Server 2007 sites.
To resolve consecutive incremental crawl failures. In rare cases, if an incremental crawl fails one hundred consecutive times at any level in a repository, the index server removes the affected content from the index.
Crawl rules have been added, deleted, or modified.
To repair a corrupted index.
The search services administrator has created one or more server name mappings.
The account assigned to the default content access account or crawl rule has changed.
Also full crawls are performed automatically (when an Incremental crawl is requested) in the following scenarios (FTA):
A SSP administrator stopped the previous crawl.
A content database was restored from backup.
A farm administrator has detached and reattached a content database.
A full crawl of the site has never been done.
The change log does not contain entries for the addresses that are being crawled. Without entries in the change log for the items being crawled, incremental crawls cannot occur.
The account assigned to the default content access account or crawl rule has changed.
To repair a corrupted index.
HTH, Dan